Future motor vehicle brake systems are expected to increasingly have wheel brakes which can be activated electromechanically. Known wheel brakes which can be activated electromechanically comprise, in addition to the electromechanical actuator, an electronic open-loop and closed-loop control unit (wheel control unit, WCU). A data processing program (application/software) which determines, from various input data items, control signals for actuating the electromechanical actuator in order to generate the desired braking force, is usually executed in the electronic open-loop and closed-loop control unit.
The international patent application WO 2008/155341 A1 discloses a combined brake system with wheel brakes which can be activated hydraulically on a first axle and wheel brakes which can be activated electromechanically on a second axle. In this context, an electrohydraulic open-loop and closed-loop control unit is assigned to the first vehicle axle, and an electronic open-loop and closed-loop control unit is assigned to each electromechanical actuator of the second axle. The open-loop and closed-loop control units are connected to one another via data buses.
When there is a classic functional division in respect of the hardware components and software components, the assigned processor-based control unit (WCU) operates on the basis of sensor-support control. This control is capable of performing a maximum number of iterations (calculations) per second. For example, an iteration rate of 200 s−1 corresponds to a software loop time of 5 ms. The actuation and closed-loop control processes usually occur with the timing pattern of this software loop time.
Known purely electrohydraulic brake systems usually have a central electrohydraulic open-loop and closed-loop control unit (HECU), for example an ESP assembly (ESP: electronic stability program), which actuates valves on the basis of their software loop time in order to control the brake pressure and therefore decelerate the vehicle.
Combined brake systems with wheel brakes (EMB) which can be activated hydraulically and electromechanically, such as, for example, the brake system disclosed in WO 2008/155341 A1, have, on the other hand, for example an ESP assembly as a central controller (master) which transmits communication messages to other nodes (slaves) of the network (for example to the WCU) on the basis of the (its) software loop time, and as it were predefines the setpoint variables (for example for the braking force which is to be set) by “remote control”. In this case, the valves (braking force) are no longer controlled directly (centrally) on a wheel-specific basis. Instead, the ESP assembly transmits the setpoint tension force to the EMB using communication messages, after which the respective EMB autonomously adjusts the actual tensioning force correspondingly. The electric brake is therefore part of a composite system.
An obvious disadvantage of such brake systems in the prior art is the loose chronological relationship between the software loop times of the ESP central controller (master) and the EMB (slave). If these are not matched to one another, a message which is sent by the central controller and received by the EMB can, under certain circumstances, fail to be taken into account for a certain time before the EMB follows the changed tensioning force setpoint value since the current iteration loop (of the software) still has to be ended. This delay has to be considered relevant for safety when there is a sudden braking request: the braking process can, for example, be initiated with an overall delay in a hazardous situation. In other cases, the tensioning force setpoint values can be set with a delay and at different times, as a result of which the stability of the vehicle may be compromised during the braking process.
The problems which are relevant for safety also arise during the sending of the tensioning-force actual values which are logically already present in the form of messages after transmission from the EMB to the ESP central controller but which firstly have to wait, owing to the less than optimum time of reception, for the ending of the current software loop of the central controller before they are taken into account by the ESP software. The highly clocked iteration rate of 200 or more calculations per second are effectively reduced in terms of the order of magnitude of the additional lag times along the forward channel and back channel within the grouping of a system with distributed open-loop and closed-loop control units without a fixed chronological relationship between the distributed nodes. The deceleration capability of the vehicle decreases accordingly.
The invention is therefore based on the object of making available an electronic brake system in which the timing of one electronic open-loop and closed-loop control unit which is assigned to an electromechanical actuator and that of a further open-loop and closed-loop control unit of the brake system are coordinated. Furthermore, an associated operating method and a motor vehicle with such a brake system are intended to be made available.
With respect to the brake system, the above-mentioned object is achieved according to the invention in that the loop time of the data processing program for performing open-loop and/or closed-loop control of the electromechanical actuator can be changed by means of the databus.
The terms “loop time” and “software loop time” are used according to the invention to refer to the transit time of a (main) loop of the application (software) of the corresponding open-loop and/or closed-loop control unit. Such a “loop time” is usually in the range of several milliseconds (for example 10 ms).
Advantageous refinements of the invention are the subject matter of the dependent claims.
The invention is based on the idea that in brake systems which are constructed according to the master/slave principle and in which a master unit communicates with a plurality of slaves which are assigned to brakes which can be activated electromechanically and in which the slaves, as it were, independently bring about the application of the brakes at the instigation of messages from the master, the time period between the transmission of the setpoint tensioning force by the master and the setting of the tensioning force by the respective slave should be kept as low as possible. Given an excessively long time interval between the times at which the master unit determines the setpoint tensioning forces and the times at which said forces are set by the slaves, adjustment of the tensioning forces with a minimum phase offset which is as constant as possible is made more difficult to an extreme degree. In extremely hazardous situations in which a rapid time sequence of different tensions has to be set, the actual tensioning forces and setpoint tensioning forces can get out of phase to such an extent that the vehicle cannot be stabilized enough or be brought to a standstill in good time.
The messages of the master unit should therefore arrive at the slaves within the software loop time at which they can be evaluated and processed substantially without delay, that is to say as it were in a setpoint arrival time window. The dispatch time of the messages of the master and the setpoint arrival time window should therefore be, as continuously as possible, “in phase” with one another. Such a rigid phase relationship is usually not present during normal and unsynchronized operation of the master and slaves.
As has now been recognized, such synchronization can be achieved by virtue of the fact that the software loop time of the respective slave can be changed, over one or more cycles, specifically until the arrival of the messages of the master unit occurs at the setpoint arrival time window of the slave or slaves. This adaptation should take place via a databus which connects the master unit to the respective slave unit. That is to say the software loop time of an electronic open-loop and/or closed-loop control unit which is assigned to an electromechanical actuator should be capable of being changed by means of the databus, in particular the CAN databus. However, other means which are external in relation to the slave unit can also be applied and are conceivable. For example, a separate clock line or separate trigger line, which, for example, conducts cyclically changing patterns and therefore impresses the time window, can be used.
It is therefore proposed, for example, to carry out synchronization of the software loop times of nodes in a distributed system with respect to a common reference. As a result, a certain phase rigidity is achieved within a specific framework.
In one preferred embodiment of the brake system, the electronic open-loop and/or closed-loop control unit (WCU) which is assigned to the electromechanical actuator comprises a means which adapts the loop time of the data processing program for performing open-loop and/or closed-loop control of the electromechanical actuator to the arrival of messages which have a time stamp and are received via the databus. The arrival time of the messages can be positioned by these means in a setpoint arrival time window of the WCU, which ensures immediate processing of the messages. It is therefore possible, when setpoint tensioning forces are transmitted, to adjust said forces immediately and the actual tensioning force values are transmitted back to the central open-loop and closed-loop control unit immediately hereafter. This means can comprise, for example, the execution of a difference measurement, wherein the chronological offset between the actual arrival time and the ideal position within the setpoint arrival time window is measured.
The messages with a time stamp advantageously additionally comprise further data. In this way, these additional data do not have to be transmitted by separate messages, with the result that the number of messages which have to be transmitted in a certain time interval is kept as low as possible.
For effective communication and synchronization, the messages with a time stamp are advantageously sent periodically, in particular at equidistant times, by the further open-loop and/or closed-loop control unit (ECU). Data messages can be defined unambiguously in terms of content, identifier and sending time, and therefore function as a time stamp.
The electronic open-loop and/or closed-loop control unit (WCU), which is assigned to the electromechanical actuator, itself advantageously sends a message, in particular exclusively, immediately after the reception of a message with a time stamp. This measure allows the time interval between the dispatch of this message and the next reception of a message with a time stamp to be kept as long as possible, which permits collisions of these messages with one another to be avoided.
In one preferred embodiment of the brake system, the loop time of the data processing program for performing open-loop and/or closed-loop control of the electromechanical actuator is shortened or lengthened as a function of the received time stamps for a predefined number of loops. As a result, the setpoint reception window can be chronologically shifted with respect to the time stamps, i.e. to a certain extent the setpoint reception window can be wound forward and back with respect to the time stamps. The application loop time which is lengthened or shortened in a time-limited fashion is advantageously varied here in terms of a chronological framework which can be ignored with respect to the control: the loop time is preferably lengthened or shortened by significantly less than 10% in each case.
The further open-loop and/or closed-loop control unit is advantageously a central electronic or electrohydraulic open-loop and/or closed-loop control unit which is suitable for directly or indirectly controlling all the wheel brakes.
The further, in particular the central, open-loop and/or closed-loop control unit as well as each of the electronic open-loop and/or closed-loop control units assigned to the electromechanical actuators each preferably have separate intelligence and each comprise at least one microcomputer. The electronic open-loop and/or closed-loop control units (WCU) which are assigned to the electromechanical actuators are advantageously each arranged in the vicinity of the associated wheel.
The brake system preferably comprises wheel speed sensors whose signals are fed to the central electronic or electrohydraulic open-loop and/or closed-loop control unit (ECU) and/or the electronic open-loop and closed-loop control units (WCU) of the electromechanical actuators.
The brake system of this invention advantageously includes a central electronic or electrohydraulic open-loop and/or closed-loop control unit (ECU) and at least two wheel brakes which can each be activated by an electromechanical actuator and each have an electronic open-loop and/or closed-loop control unit (WCU) which is assigned to the electromechanical actuators, wherein each electronic open-loop and/or closed-loop control unit (WCU) which is assigned to the electromechanical actuators is indirectly or directly connected to the central open-loop and/or closed-loop control unit (ECU) via a databus, in particular a CAN (Controller Area Network) databus. The central open-loop and/or closed-loop control unit (ECU) at least temporarily actuates at least one of the electronic open-loop and/or closed-loop control unit (WCU) which is assigned to the electromechanical actuators.
According to one advantageous development, the brake system comprises two wheel brakes which are assigned to a second vehicle axle and can be activated by electromechanical actuators, as well as two wheel brakes which are assigned to a first vehicle axle and can be activated electromechanically or hydraulically, wherein the central electronic or electrohydraulic open-loop and/or closed-loop control unit (ECU) is designed to perform closed-loop control of the wheel brakes which are assigned to the first vehicle axle.
The central open-loop and/or closed-loop control unit (ECU) is preferably embodied as an electrohydraulic open-loop and closed-loop control unit (HECU). Said electrohydraulic open-loop and closed-loop control unit advantageously has a device for carrying out an anti-lock brake function (ABS) and/or a vehicle movement dynamics stability function (ESP). Corresponding braking force prescriptions are transmitted from the central open-loop and/or closed-loop control unit (ECU) via the databus (CAN bus) to the electronic open-loop and/or closed-loop control units (WCU) which are assigned to the electromechanical actuators.
With respect to the method, the above-mentioned object is achieved according to the invention in that the loop time of the data processing program for performing open-loop and/or closed-loop control of the electromechanical actuator can be changed by means of the databus. The loop time is advantageously adapted to messages with time stamps which are received via the databus.
The software of a mechatronic wheel brake actuator (electromechanical brake, EMB) is influenced in its chronological processing sequence preferably via external means, for example the bus communication.
The application loop time of the controlling software of the electromechanical brake is advantageously synchronized with fixed time stamps in the form of received CAN messages. These CAN messages are sent by the sender periodically or even at strictly equidistant times.
According to one preferred development of the method of this invention, the electromechanical brake sends its CAN message immediately in reaction to a received CAN message. This prevents a collision with the next reception message and therefore ensures that the messages of the sender which function as time stamps are sent at equidistant times (owing to the now mentioned freedom from collision). The possibility of arbitrating between messages, which is permitted according to the CAN specification, is not used.
In one advantageous embodiment of the method, messages with a time stamp are sent recurrently by the central open-loop and/or closed-loop control unit (ECU), in particular at a permanently predefined time interval, to the electronic open-loop and/or closed-loop control unit (WCU) which is assigned to the electromechanical actuator. Said electronic open-loop and/or closed-loop control unit (WCU) particularly preferably evaluates the arrival times of the messages in order to bring about synchronization with the central open-loop and/or closed-loop control unit (ECU).
The central open-loop and/or closed-loop control unit (ECU) at least temporarily actuates the electronic open-loop and/or closed-loop control unit (WCU) which is assigned to the electromechanical actuator, for example the central open-loop and/or closed-loop control unit (ECU) prescribes setpoint values for the braking force to be adjusted to the electronic open-loop and/or closed-loop control unit (WCU) which is assigned to the electromechanical actuator.
With respect to the motor vehicle, the above-mentioned object is achieved according to the invention with an above-mentioned brake system.
The advantages of the invention are, in particular, the fact that for changing the software loop time of the data processing program for performing open-loop and/or closed-loop control of an electromechanical actuator by means of a databus essentially delay-free and lag-time-free processing of transmitted messages can take place and therefore an immediate reaction to the content of these messages can occur.
Identical parts are provided with the same reference symbols in all the figures.